A comprehensive epidemiologic study of mental retardation and developmental disability is proposed, to be based on a national cohort of Netherlands births for the years 1944-1947. The investigators will avail themselves of a unique body of data, which includes systematic information on births and deaths, as well as on mental disorder and health status in 400,000 survivors, to carry out cohort and prevalence studies. Cohort analysis will make it possible for the first time to measure the contribution of survival to changes in prevalence. Studies will be both descriptive and analytic. Biological and social influences in the incidence, persistence and expression of all levels of mental retardation will be explored.